Hot-rolled steel sheets with a tensile strength of 590 MPa or lower have been used for components, such as automobile and truck frames, which require formability and tensile fatigue properties because conventional 780 MPa grade steel is difficult to shape. Furthermore, the thickness of a 780 MPa grade steel sheet is, as a matter of course, smaller than that of a 590 MPa grade steel sheet. Consequently, the tensile fatigue properties of the conventional 780 MPa grade steel are insufficient when used for such components. However, in recent years, in order to improve the crashworthiness of automobiles, an increase in the strength of steel sheets for automobiles has been promoted, and use of 780 MPa grade steel for portions requiring tensile fatigue properties has come under study. The formability required for such components includes elongation and stretch-flangeability.
Examples of the method for improving elongation includes a technique using retained austenite, which is disclosed in Patent Document 1. However, retained austenite degrades stretch-flange formability. It is known that stretch-flangeability improves as the difference in hardness between the matrix and the other phases decreases. In retained austenite steel, the second phase is harder than the ferrite matrix and the difference in hardness between the second phase and the ferrite matrix is large. Thus, degradation in stretch-flange formability has been a problem. Meanwhile, in tempered martensite and bainitic single phase steel, stretch-flange formability is good because of a small difference in hardness between the matrix and the second phase, but ductility is low. Therefore, in order to achieve both ductility and stretch-flangeability, multiple phase steel is required in which the difference in hardness between the matrix and the second phase is small. Techniques regarding multiple phase steel sheets are disclosed in which the ferrite phase is precipitation-hardened by precipitates containing Ti, Mo, and W (Patent Document 2) and by precipitates containing Ti and Mo (Patent Document 3) so that the difference in hardness between the matrix and the bainite second phase is decreased. Furthermore, these patent documents are characterized by the fact that, while TiC can be easily coarsened by heat treatment, precipitates including Ti and Mo are inhibited from being coarsened. However, Mo is expensive compared with Ti, Nb, and V, which are carbide-forming elements, and moreover, in steel sheets which are produced by quenching followed by air cooling, or by holding followed by quenching, only about 50% or less of the Mo content in steel is precipitated, giving rise to a problem of cost increase.
Under these circumstances, there has been a demand for a technique which can increase the strength while satisfying the requirements for ductility and stretch-flangeability without using expensive Mo, but using a less expensive element, such as Ti.
Furthermore, Patent Document 4 discloses a technique on a steel sheet composed of phases of ferrite, which is precipitation-hardened by TiC, and bainite. According to an example in this patent document, at a sheet thickness of 2.9 mm, the tensile strength is 740 N/mm2, the product (tensile strength)×(elongation) is 18,000 N/mm2·% or more, and the product of hole expanding ratio and tensile strength, (tensile strength)×(hole expanding ratio), which is an index for stretch-flangeability, is 40,000 N/mm2 or more. However, the tensile fatigue properties are not necessarily sufficient.
As a technique for improving fatigue properties, Patent Document 5 discloses a technique in which elongation and fatigue properties are improved by controlling the compositional fractions in a surface layer and an internal layer. However, this patent document does not mention any measures for improving stretch-flangeability.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 7-62485
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-321739
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2004-339606
Patent Document 4: Japanese Unexamined Patent Application Publication No. 8-199298
Patent Document 5: Japanese Unexamined Patent Application Publication No. 11-241141